System.NullReferenceException: 'Object reference not set to an instance of an object.' problem with blazor server app

I'm making my first blazor application and I'mrunning into an error, when I'm trying to get a specific User by Id.

for some contexts I'm using a Blazor server as frontend with a .NET 8 API as a backend

Blazor page code:

@page "/User"@page "/User/{Id:int}"@rendermode InteractiveServer@using System.Net.Http@using Models@using System.Text.Json;@using System.Text@using System.Security.Cryptography;@inject HttpClient HttpClient@inject NavigationManager NavManager@if (Id == null){<h3>User Add</h3>}else{<h3>User Edit</h3>}<form method="post" @onsubmit="AddUser" @formname="UserAdd" class="mt-4"><AntiforgeryToken /><div class="mb-3"><label for="name" class="form-label">Name:</label><InputText type="text" @bind-Value="user!.Name" class="form-control" required /></div><div class="mb-3"><label for="Company" class="form-label">Company:</label><InputText type="text" @bind-Value="user!.Company" class="form-control" required /></div><div class="mb-3"><label for="Department" class="form-label">Department:</label><InputText type="text" @bind-Value="user!.Department" class="form-control" required /></div><div class="mb-3"><label for="email" class="form-label">Email:</label><InputText type="email" @bind-Value="user!.Email" class="form-control" required /></div><div class="mb-3"><label for="Password" class="form-label">Password:</label><InputText type="password" @bind-Value="user!.Password" class="form-control" required /></div><button type="submit" class="btn btn-primary">Submit</button></form>@code {    [Parameter]    public int? Id { get; set; }    [SupplyParameterFromForm]    public User? user { get; set; }    protected override async Task OnInitializedAsync()    {        if (Id != null)        {            try{                var response = await HttpClient.GetAsync($"https://localhost:7183/api/User/{Id}");                response.EnsureSuccessStatusCode();                var json = await response.Content.ReadAsStringAsync();                user = JsonSerializer.Deserialize<User>(json, new JsonSerializerOptions { PropertyNameCaseInsensitive = true });            }            catch (Exception ex)            {                // Handle error                Console.WriteLine("Error getting user: " + ex.Message);            }        }        else        {            user ??= new();        }    }    void AddUser()    {        try        {            byte[] salt = GenerateSalt();            user.Password = HashPassword(user.Password, salt);            var client = new HttpClient();            client.BaseAddress = new Uri("https://localhost:7183/api/User");            var json = JsonSerializer.Serialize(user);            var content = new StringContent(json, Encoding.UTF8, "application/json");            var response = client.PostAsync("User", content).Result;            NavManager.NavigateTo("/UserOverview");        }        catch (Exception ex)        {            // Handle error            Console.WriteLine("Error adding user: " + ex.Message);        }    }    private string HashPassword(string password, byte[] salt)    {        using (Rfc2898DeriveBytes pbkdf2 = new Rfc2898DeriveBytes(password, salt, 10000))        {            byte[] hash = pbkdf2.GetBytes(32); // 32 bytes for SHA256            byte[] hashBytes = new byte[36]; // 32 bytes for hash + 4 bytes for salt            Array.Copy(salt, 0, hashBytes, 0, 4);            Array.Copy(hash, 0, hashBytes, 4, 32);            return Convert.ToBase64String(hashBytes);        }    }    private byte[] GenerateSalt()    {        byte[] salt = new byte[4]; // 4 bytes for salt        using (RNGCryptoServiceProvider rngCsp = new RNGCryptoServiceProvider())        {            rngCsp.GetBytes(salt);        }        return salt;    }}

And the API methode i have:

group.MapGet("/{id}", async Task<Results<Ok<User>, NotFound>> (int id, DataContext db) =>{    return await db.User.AsNoTracking()        .FirstOrDefaultAsync(model => model.Id == id)        is User model            ? TypedResults.Ok(model)            : TypedResults.NotFound();}).WithName("GetUserById").WithOpenApi();

after the line:var response = await HttpClient.GetAsync($"https://localhost:7183/api/User/{Id}");

i get this error message:

System.NullReferenceException  HResult=0x80004003  Message=Object reference not set to an instance of an object.  Source=CalibrationReportingToolFrontend  StackTrace:   at CalibrationReportingToolFrontend.Components.Pages.OtherPages.UserAdd.BuildRenderTree(RenderTreeBuilder __builder)

I thought maybe the Id i was looking for doesn't exist in the DB so i tried this:var response = await HttpClient.GetAsync("https://localhost:7183/api/User/6");

Beside that i also tested my API endpoint and this seems to be working as expected